The present invention relates to a communication system and a communication method for performing data communication of discrete multi-tone modem type, for example, between a plurality of data communication units through a telephone line.
In recent years, the xDSL communication system which uses the existing telephone copper cable, including the ADSL (Asymmetric Digital Subscriber Line) communication system, the HDSL (High-bit-rate Digital Subscriber Line) communication system and the SDSL communication system for performing a high-speed digital communication of several mega bits per second, have been closely watched. The xDSL communication system is called the DMT (Discrete Multi-Tone) modem system. This system is standardized in T1.413, etc. of ANSI.
This digital communication system, especially in the case where the xDSL transmission path and the ISDN transmission path of the half-duplex ISDN communication system are bound together as an aggregated line or otherwise placed adjacently to each other, poses the problem that the xDSL communication through the xDSL transmission path is affected by interference noises from the ISDN transmission path or other lines and decreases in speed. For solving this problem, various devices are introduced.
FIG. 14 shows the interference noises of an ISDN transmission path 2 from a central office (CO) 1, which affects an ADSL transmission path 3 constituting a xDSL transmission path bound with the ISDN transmission path 2 midway as an aggregated line.
When viewed from the ADSL terminal equipment (ATU-R; ADSL transceiver unit, remote terminal end) 4 constituting a communication unit at a terminal of the ADSL communication system, the interference noise transmitted through the ADSL transmission path 3 by the office equipment (ISDN LT) 7 of the ISDN transmission system is called the FEXT (far-end crosstalk) noise, while the interference noise transmitted through the ADSL transmission path 3 by the terminal equipment (ISDN NT1) 6 of the ISDN transmission system is called the NEXT (near-end crosstalk) noise. Especially, these noises are transmitted to the ADSL terminal equipment (ATU-R) 4 through the ADSL transmission path 3 which is coupled with the ISDN transmission path 2 midway as an aggregated line.
When viewed from the ADSL office equipment (ATU-C: ADSL transceiver unit, central office end) 5 constituting the office equipment of the ADSL communication system, on the other hand, the result is opposite from the case viewed from the ADSL terminal equipment (ATU-R) 4. In such a case, the interference noise transmitted by the office equipment (ISDN LT) 7 of the ISDN transmission system constitutes the NEXT noise, while the interference noise transmitted by the terminal equipment (ISDN NT1) 6 of the ISDN transmission system makes up the FEXT noise.
In an overseas ISDN communication system in U.S.A., for example, which is full-duplexed, the up and down transmissions are performed at the same time. When viewed from the ADSL terminal equipment (ATU-R) 4, therefore, the NEXT noise generated by the terminal equipment (ISDN NT1) 6 of the ISDN transmission system nearer to the ADSL terminal equipment (ATU-R) 4 is controlling, i.e. has a larger effect.
Therefore, during the training period of the ADSL modem (not shown) installed at the ADSL terminal equipment 4, the characteristic of the NEXT noise components having a large effect is measured, and the number of transmission bits and the gain of each channel meeting the noise characteristic are determined by bit mapping. Further, in order to improve the transmission characteristics, the coefficients of the time domain equalizer (TEQ) for adaptive equalization in time domain and the frequency domain equalizer (FEQ) for adaptive equalization in frequency domain are converged and determined, so that a set of coefficient tables for NEXT noises are provided for each of TEQ and FEQ.
Although this measure eliminates the problem in the aforementioned digital communication systems, the half-duplex communication system TCM-ISDN employed in Japan as an existing ISDN communication system, in which the up and down data transmission are switched by time division like Ping-Pong, poses the problem that in the case where the half-duplex transmission path and other transmission path are adjacently placed to each other on an aggregated line or the like, the NEXT noises and the FEXT noises from the half-duplex transmission path have an effect alternately on the communication terminals connected to the other transmission paths adjacent to the half-duplex transmission path.
In the Japanese ADSL system, therefore, a method is proposed in which the bit map is switched in accordance with the FEXT and NEXT sections of the TCM-ISDN interference noises (xe2x80x9cG. lite: Proposal for draft of Annex of G. litexe2x80x9d, ITU-T, SG-15, Waikiki, Hi. 29 June-3 July 1998, Temporary Document WH-047).
FIG. 15 shows an outline of a digital communication system using the digital communication equipment employing the method described above. In FIG. 15, numeral 11 designates a central office (CO) which controls the TCM-ISDN communication and the ADSL communication, numeral 12 designates a TCM-ISDN transmission path for the TCM-ISDN communication, numeral 13 designates an ADSL transmission path for the ADSL communication, numeral 14 designates an ADSL terminal equipment (ATU-R; ADSL transceiver unit, remote terminal end) such as a communication modem which performs the ADSL communication with other not shown ADSL communication terminal equipment through the ADSL transmission path 13, numeral 15 designates an ADSL office equipment (ATU-C; ADSL transceiver unit, central office end) which controls the ADSL communication within the central office 11, numeral 16 designates a TCM-ISDN terminal equipment (TCM-ISDN NT1) such as a communication modem which performs the TCM-ISDN communication with other not shown TCM-ISDN terminal equipment through the TCM-ISDN transmission path 12, numeral 17 designates a TCM-ISDN office equipment (TCM-ISDN LT) which controls the TCM-ISDN communication in the central office 11, and numeral 18 designates a sync controller which synchronizes the communication between the TCM-ISDN office equipment (TCM-ISDN LT) 17 and the ADSL office equipment (ATU-C) 15. The sync controller 18 may alternatively be installed in the TCM-ISDN office equipment (TCM-ISDN LT) 17 or the ADSL office equipment (ATU-C) 15.
As described above, when viewed from the ADSL terminal equipment (ATU-R) 14, as shown in FIG. 15, the interference noise transmitted by the TCM-ISDN office equipment (TCM-ISDN LT) 17 providing a far half-duplex communication system through the TCM-ISDN transmission path 12 and the ADSL transmission path 13 which are adjacent on an aggregated line is called the xe2x80x9cFEXT noisexe2x80x9d, while the interference noise transmitted by the TCM-ISDN terminal equipment (TCM-ISDN NT1) 16 constituting a near half-duplex communication system through the TCM-ISDN transmission path 12 and the ADSL transmission path 13 which are adjacent on an aggregated line is called the xe2x80x9cNEXT noisexe2x80x9d.
When viewed from the ADSL office equipment (ATU-C) 15, on the other hand, the case is opposite to the view from the ADSL terminal equipment (ATU-R) 14, and the interference noise transmitted by the office equipment (ISDN LT) 17 of the ISDN transmission system constituting the near half-duplex communication system is the NEXT noise, while the interference noise transmitted by the terminal equipment (ISDN NT1) 16 of the ISDN transmission system making up a far half-duplex communication system constitutes the FEXT noise.
FIG. 16 shows a functional configuration of a transmission unit or a dedicated transmitter (hereinafter referred to as the transmission system) such as a communication modem of the ADSL office equipment (ATU-C; ADSL transceiver unit, central office end) 15 of the digital communication system. On the other hand, FIG. 17 shows a functional configuration of a receiving unit or a dedicated receiver (hereinafter referred to as the receiving system) such as a communication modem of the ADSL terminal equipment (ATU-R) 14 of the digital communication system.
In FIG. 16, numeral 41 designates a multiplex/sync controller, numerals 42 and 43 designate cyclic redundancy check (crc) units, numerals 44 and 45 designate scramble forward error correction units (Scram and FEC), numeral 46 designates an interleaver, numerals 47 and 48 designate rate converters, numeral 49 designates a tone ordering unit, numeral 50 designates a constellation encoder and gain scaling unit, numeral 51 designates an inverse discrete Fourier transform unit (IDFT), numeral 52 designates an input parallel/serial buffer, and numeral 53 designates an analog processing and D/A converter (DAC).
In FIG. 17, numeral 141 designates an analog processing and A/D converter (ADC), numeral 142 designates a time domain equalizer (TEC), numeral 143 designates an input serial/parallel buffer, numeral 144 designates a discrete Fourier transform unit (DFT), numeral 145 designates a frequency domain equalizer (FEQ), numeral 146 designates a constellation encoder and gain scaling unit, numeral 147 designates a tone ordering unit, numerals 148, 149 designate rate converters, numeral 150 designates a deinterleaver, numerals 151, 152 designate descramble forward error correction units (FEC), numerals 153, 154 designate cyclic redundancy check units (crc), and numeral 155 designates a multiplex/sync controller.
Now, the operation will be explained. First, the reference is made to the operation of the transmission system of the ADSL office equipment (ATU-C) 15. In FIG. 16, the transmission data are multiplexed by the multiplex/sync controller 41, and have an error detection code added thereto by the cyclic redundancy check units 42, 43, have the FEC code added thereto and subjected to the scramble processing by the scramble forward error correction units 44, 45, sometimes followed by the processing in the interleaver 46. After that, the rate is converted by the rate converters 47, 48, the tone ordering is executed by the tone ordering unit 49, the constellation data are produced by the constellation encoder and gain scaling unit 50, the inverse discrete Fourier transform is carried out by the inverse discrete Fourier transform unit 51, the digital waveform is converted into an analog waveform through the analog processing and D/A converter 53, and then the signal is applied through a low-pass filter.
An explanation will be given of the operation of the receiving system of the ADSL terminal equipment (ATU-R) 14. In FIG. 17, the analog processing and A/D converter 141 applies the received signal through a low-pass filter, and converts the analog waveform into a digital waveform through the A/D converter, followed by the time domain adaptive equalization through a time domain equalizer (TEQ) 142.
Then, the data subjected to the time domain adaptive equalization are converted from serial to parallel data through the input serial/parallel buffer 143, subjected to the discrete Fourier transform in the discrete Fourier transform unit (DFT) 144, and then subjected to the frequency domain adaptive equalization by the frequency domain equalizer (FEQ) 145.
The constellation data are reproduced by the constellation encoder and gain scaling unit 146, converted into the serial data by the tone ordering unit 147, have the rate converted in the rate converters 148, 149, subjected to the descramble processing and FEC by the descramble and forward error correction unit 151, and in some cases, after being deinterleaved by the deinterleaver 150, subjected to FEC and descramble processing by the descramble and forward error correction unit 152. After the processing in the cyclic redundancy check unit 153, 154, the data are reproduced by the multiplex/sync controller 155.
In the process, the sync controller 18 of the central office (CO) 11 synchronizes the transmission timing between the TCM-ISDN office equipment (TCM-ISDN LT) 17 and the ADSL office equipment (ATU-C) 15. Thus, the ADSL terminal equipment (ATU-R) 14 can recognize the timing of generation of the NEXT noise and FEXT noise.
Specifically, the ADSL terminal equipment (ATU-R) 14, by the synchronization between the TCM-ISDN communication and the ADSL communication, determines that the NEXT noise is generated in the received data or the signal received through the ADSL transmission path 13 during a predetermined time when the data are transmitted up the TCM-ISDN transmission path 12 at a known timing. On the other hand, during a predetermined time when the data are transmitted down the TCM-ISDN transmission path 12 at a known timing, the generation of the FEXT can be similarly recognized in the data received through the ADSL transmission path 13.
In the Japanese ADSL system, as shown in FIG. 18, the bit map A and the bit map B are assigned to the FEXT sections and the NEXT sections, respectively, and in the rate converters 47, 48 of FIG. 16, the bit distribution is increased in FEXT section where there is a less noise, and the bit distribution is reduced in the NEXT section where there is a large noise. As a result, the transmission rate can be improved as compared with the conventional case in which the bit distribution is determined only by the NEXT section.
FIG. 19 shows the manner in which the data received at uniform rate (64 kbps in the calculation example below) are assigned to the bit map A and the bit map B at the time of transmission. First, the data sent in at uniform rate are stored in the form of fixed bits in units of symbols. These data are converted into bits for the bit map A and the bit map B by a rate converter. An integer multiple is not involved, however, because the interval of the transmitted symbols is 246 xcexcs for the ISDN period of 2.5 ms.
Thus, as shown in FIG. 20, with 34 periods (=345 symbols, 85 ms) as one unit (hyperframe), only the FEXT section in the hyperframe where the symbols are filled up is defined as a bit map A, and the other portions as a bit map B (in the drawing, SS and ISS indicate sync signals). Whether each DMT symbol is associated with bit map A or bit map B is determined from the following equations. (In the equations below, the DMT symbol No. is assumed to be Ndmt)
Transmission from ATU-C to ATU-R
S=272xc3x97Ndmt mod 2760
if {(S+271 less than a) or (S greater than a+b)} then [bit map A symbol]
if {(S+271 greater than =a) and (S less than =a+b)} then [bit map B symbol]
where a=1243, and b=1461.
Transmission from ATU-R to ATU-C
S=272xc3x97Ndmt mod 2760
if {(S greater than a) and (S+271 less than a+b)} then [bit map A symbol]
if {(S less than 32 a) or (S+271 greater than 32 a+b)} then [bit map B symbol]
where a=1315, and b=1293.
An example of calculation for determining the bit assignment for the single bit map using only the bit map A to assign data is shown below.
Number of bits of 1 DMT symbol (before rate conversion)
=(transmission rate)xc3x97(transmission time)/(total number of symbols (except for ISS (inverse sync symbol) and SS (sync symbol))
=64 kbpsxc3x9785 ms/340
=16 bits
Number of bits of bit map A
=(transmission rate)xc3x97(transmission time)/(total number of symbols of bit map A (except for ISS (inverse sync symbol) and SS (side A sync symbol))
=64 kbpsxc3x9785 ms/126
=43.175
Thus, the bit map A is assumed to be equal to 44 bits. Further, because of the single bit map (using only the bit map A), the bit map B is set to zero bit.
The following is an example of calculation for determining the bit assignment for the dual bit map using both the bit map A and the bit map B.
Number of bits of 1 DMT symbol (before rate conversion)
=(transmission rate)xc3x97(transmission time)/(total number of symbols (except for ISS (inverse sync symbol) and SS (sync symbol))
=64 kbpsxc3x9785 ms/340
=16 bits
This calculation example assumes that the number of bits of the bit map B is 3.
Number of bits of bit map A
=((transmission rate)xc3x97(transmission time) (number of bits per symbol of bit map B)xc3x97(number of symbols of bit map B (except for ISS (inverse sync symbol) and SS (side A sync symbol)))/(number of symbols of bit map A (except for ISS (inverse sync symbol) and SS (side A sync symbol))
=(64 kbpsxc3x9785 msxe2x88x923xc3x97214)/126
=38.079 bits
Thus, the bit map A has 39 bits.
When changing the bit distribution by a rate converter as described above, the data are output only after being stored to some degree in the rate converter at the transmitting or receiving end. Therefore a delay time occurs in the rate converter. Further, with a single bit map, the transmission data are assigned as fully in the bit map A as possible in each hyperframe. In some cases, therefore, the data of a given period may be assigned to the bit map A of a subsequent period. A further delay time is caused by such data. Even with a dual bit map, bits are assigned in the bit map A and the bit map B of the hyperframe as fully as possible. In some cases, therefore, the data of a given period may be assigned to a subsequent period, which causes an additional delay time for the particular data. In this conventional system, an excessively large delay is a problem.
Accordingly, the object of the present invention is to provide a communication system and a communication method capable of suppressing the delay.
According to one aspect of the present invention, there is provided a communication system for setting in one period the data transmission time suitable for data transmission and the quasi-data transmission time other than the data transmission time in accordance with the transmission path, and communicating by multiplexing first data and second data, wherein bits are assigned in such a manner that the first data for one period can be transmitted during the data transmission time of one period, and also in such a manner that the second data for a predetermined period can be transmitted in the portion of the data transmission time for the predetermined period not assigned the first data.
According to another aspect of the present invention, there is provided a communication system for setting in one period the data transmission time suitable for data transmission and the quasi-data transmission time other than the data transmission time in accordance with the transmission path, and communicating by multiplexing first data and second data, wherein bits are assigned in such a manner that the first data for one period can be transmitted during the data transmission time and the quasi-data transmission time of one period, and also in such a manner that the second data for a predetermined period can be transmitted in the portion of the data transmission time and the quasi-data transmission time for the predetermined period which is not assigned the first data.
According to still another aspect of the present invention, there is provided a communication system for setting in one period the data transmission time suitable for data transmission and the quasi-data transmission time other than the data transmission time in accordance with the transmission path, and communicating by multiplexing first data and second data, wherein bits are assigned in such a manner that the first data for one period can be transmitted during the data transmission time of one period, and also in such a manner that the second data for a predetermined period can be transmitted in the portion of the data transmission time for the predetermined period which is not assigned the first data, such data are received of which the whole first data for one period are reproduced based on the first data assigned to the data transmission time for one period, and the whole second data for a predetermined period are reproduced based on the received second data assigned to the portion of the data transmission time of the predetermined period.
According to still another aspect of the present invention, there is provided a communication system for setting in one period the data transmission time suitable for data transmission and the quasi-data transmission time other than the data transmission time in accordance with the transmission path, and communicating by multiplexing first data and second data, wherein bits are assigned in such a manner that the first data for one period can be transmitted during the data transmission time and the quasi-data transmission time of one period, and also in such a manner that the second data for a predetermined period can be transmitted in the portion of the data transmission time and the quasi-data transmission time for the predetermined period which is not assigned the first data, such data are received of which the whole first data for one period are reproduced based on the first data assigned to the data transmission time and the quasi-data transmission time for one period, and the whole second data for a predetermined period are reproduced based on the received second data assigned to the portion of the data transmission time and the quasi-data transmission time of the predetermined period.
According to still another aspect of the present invention, there is provided a communication method for setting in one period the data transmission time suitable for data transmission and the quasi-data transmission time other than the data transmission time in accordance with the transmission path, and communicating by multiplexing first data and second data, wherein bits are assigned in such a manner that the first data for one period can be transmitted during the data transmission time of one period, and also in such a manner that the second data for a predetermined period can be transmitted in the portion of the data transmission time for the predetermined period not assigned the first data.
According to still another aspect of the present invention, there is provided a communication method for setting in one period the data transmission time suitable for data transmission and the quasi-data transmission time other than the data transmission time in accordance with the transmission path, and communicating by multiplexing first data and second data, wherein bits are assigned in such a manner that the first data for one period can be transmitted during the data transmission time and the quasi-data transmission time of one period, and also in such a manner that the second data for a predetermined period can be transmitted in the portion of the data transmission time and the quasi-data transmission time for the predetermined period which is not assigned the first data.
According to still another aspect of the present invention, there is provided a communication method for setting in one period the data transmission time suitable for data transmission and the quasi-data transmission time other than the data transmission time in accordance with the transmission path, and communicating by multiplexing first data and second data, wherein bits are assigned in such a manner that the first data for one period can be transmitted during the data transmission time of one period, and also in such a manner that the second data for a predetermined period can be transmitted in the portion of the data transmission time for the predetermined period which is not assigned the first data, such data are received of which the whole first data for one period are reproduced based on the first data assigned to the data transmission time for one period, and the whole second data for a predetermined period are reproduced based on the received second data assigned to the portion of the data transmission time of the predetermined period.
According to still another aspect of the present invention, there is provided a communication method for setting in one period the data transmission time suitable for data transmission and the quasi-data transmission time other than the data transmission time in accordance with the transmission path, and communicating by multiplexing first data and second data, wherein bits are assigned in such a manner that the first data for one period can be transmitted during the data transmission time and the quasi-data transmission time of one period, and also in such a manner that the second data for a predetermined period can be transmitted in the portion of the data transmission time and the quasi-data transmission time for the predetermined period which is not assigned the first data, such data are received of which the whole first data for one period are reproduced based on the first data assigned to the data transmission time and the quasi-data transmission time for one period, and the whole second data for a predetermined period are reproduced based on the received second data assigned to the portion of the data transmission time and the quasi-data transmission time of the predetermined period.